1. Field of the Invention
The present invention relate to a digital TV receiver, and more particularly, to an apparatus for recovering a symbol clock from received data.
2. Discussion of the Related Art
An advanced television systems committee (ATSC) 8 VSB (Vestigial Side Band) transmission system proposed by most current digital transmission systems and a US directed digital TV transmission mode loads data only in a transmission signal to increase an effect of a frequency. That is, clock information needed for data recovery at a receiving party is not transmitted. Therefore, the same clock as that employed during the transmission should be generated among the received signals having only data to recover the data at the receiving party. A symbol clock recovery performs the role.
FIG. 1 is a block diagram illustrating a general digital TV receiver having such symbol clock recovery. Referring to FIG. 1, if a radio frequency (RF) signal modulated in a VSB mode is received through an antenna 101, a tuner 102 selects a desired channel frequency. Then, the tuner 102 converts a VSB signal of an RF band inserted in the channel frequency to a first intermediate frequency (IF) band, and outputs to an analog processor 103. The analog processor 103 performs passband filtering and gain controlling to the first IF signal outputted from the tuner 102 for converting the first IF signal into a second IF signal, and outputs to an A/D converter 104. The A/D converter 104 samples the second IF signal with a fixed frequency (i.e., the fixed frequency is different from the symbol clock frequency, and normally 25 MHz), and outputs to a phase splitter 105. That is, the data sampled at 21.52 MHz two times the frequency of the symbol clock is received at the receiving party although the outputted data from the A/D converter 104 is digital data sampled at 25 MHz.
The phase splitter 105 splits the digital signal into a passband real signal r(t)) and a passband imaginary signal (i(t)), and outputs the signal to the carrier recovery 106. At this time, for easier description, the real/imaginary signals outputted from the phase splitter 105 is named as I and Q signals, respectively.
The carrier recovery 106 converts the digital signals I and Q of the passband outputted from the phase splitter 105 to a baseband. The output signal of the carrier recovery 106 is inputted to a symbol clock recovery 107, a synchronized signal detector 108 and a digital processor 109.
At this time, the symbol clock recovery 107 recovers all the symbol clocks employed by from the carrier recovery 106 to the digital processor 109 and the synchronized signal detector 108 detects a segment sync and a field sync from the digital baseband signal.
FIG. 2 is a block diagram of the carrier recovery 106 employing a FPLL (Frequency Phase Locked Loop). That is, the carrier recovery 106 having the FPLL demodulates the passband I and Q signals outputted from the A/D converter 104 into the baseband I and Q signals for frequency and phase locking.
Referring to FIG. 2, the passband I and Q signals being digitized through the A/D converter 104 and the phase splitter 105 are inputted to a complex multiplier 201 of the carrier recovery. At this time, the real signal (r(t)) and the imaginary signal (q(t)) outputted from the phase splitter 105 is expressed as a following formula.r(t)={I(t)+p} cos(wct+Ψ)−Q(t)sin(wct+Ψ)i(t)={I(t)+p} sin(wct+Ψ)+Q(t)cos(wct+Ψ)  [Formula 1]
I(t) is a signal before a modulation and p is a pilot signal inputted to the transmitter for the carrier recovery. Also, wc is a frequency of the carrier signal existing in an input signal and Ψ is a phase of the carrier signal existing in the input signal. Q(t) is an orthogonal signal component of I(t).
Meanwhile, the complex multiplier 201 of the carrier recovery 106 multiplies the passband I and Q signals as the formula 1 by a standard carrier signals NCOI and NCOQ outputted from the NCO 205, and converts the passband I and Q signal into the baseband I and Q signals (I′(t),Q′(t)) as a following formula 2.I′(t)={I(t)+p} cos(Δwct+Ψ)−Q(t)sin(Δwct+Ψ)Q′(t)={I(t)+p} sin(Δwct+Ψ)+Q(t)cos(Δwct+Ψ)  [Formula 2]
The Δwc is a beat frequency of the carrier signal wc employed by the transmitter and the standard carrier signals NCOI and NCOQ generated from the receiver.
The I and Q signals of the baseband are outputted to a low pass filter 202 as well as to the symbol clock recovery 107 and the digital processor 109.
The low-pass filter 202 filters the low pass I and Q signals to extract the carrier and outputs to an error detector 203. That is, the carrier recovery 106 recovering the carrier needs only signals around the frequency having the pilot frequency in a band width of 6 MHz and, therefore, the low-pass filter prevents the efficiency of the carrier recovery from being reduced by removing the remaining frequency component having data component from the I and Q signals.
The error detector 203 detects remaining error of the carrier from the carrier signal, and outputs to the low-pass filter 204. That is, the remaining carrier error detected from the error detector 203 is outputted to an NCO 205 through the low-pass filter 204 to prevent errors from being accidentally detected. The NCO 205 generates new carrier signals NCOI and NCOQ and outputs to the complex multiplier 201.
If the carrier recovery is completely performed at the carrier recovery 106, Δwct and Ψ become ‘0’, and the formula 2 will be changed to a following formula 3.I′(t)=I(t)+p Q′(t)=Q(t)  [Formula 3]
The symbol clock recovery 107 performs the symbol clock recovery from the signal of the formula 3 and generates the symbol clocks employed in all digital areas of the receiver.
However, if the carrier recovery is not completely carried out in the carrier recovery 106, the symbol clock recovery 107 recovers the symbol clock from the signal of the formula 2. Thus, the symbol clock recovery is not normally performed being influenced by the frequency and the phase between the carrier signals employed by the receiver and the standard carrier signal generated from the receiver such as Δwc and Ψ.
In other words, as described in FIG. 1, the performance of the carrier recovery largely influences the performance of the symbol clock recovery in a structure the carrier recovery and the symbol clock recovery is connected. The symbol clock recovery is influenced by the remaining frequency and phase error not completely removed from the carrier recovery, and that gives bad influence on the total performance of the symbol clock recovery.
The reason why the symbol clock recovery is located at an end of the general carrier recovery is that the symbol clock recovery is designed under an assumption that the role of the carrier recovery is completed. Therefore, if the carrier recovery is not completely performed, the symbol clock recovery is not performed as well.